Most ferroelectric ceramics of commercial importance are prepared by conventional powder processing techniques. Such techniques involve mixing the precursor oxide (e.g., carbonate, nitrate, chloride, acetate, etc.) powders in the correct proportions to produce the desired composition, followed by thermally reacting the mixture to produce a new compound or mixture of new compounds. The precursor powders are converted through the thermal reaction process into a new material which is characterized by a crystal structure or mixture of crystal structures which are different from those of the precursors. In conventional powder processing the degree of homogenization and conversion to the new compound or compounds depends on particle size, how well the precursors are mixed, and the rate at which interdiffusion of the atomic species occurs under the thermal reaction conditions imposed.
The ultimate limitation to homogenization in conventional powder processing is the fineness of the precursor particle size. Interdiffusion is often not able to eliminate all the compositional differences introduced by using precursor powders of some finite particle size. After the thermal treatment some remnant trace of the precursor particles can remain in the form of microregions richer in a particular component. Enrichment of a particular species in isolated microregions therefore indicates that the donor particle for that species formerly resided in the center of the microregion richer in that particular component. For example, in the reaction, EQU BaCO.sub.3 +(1-x)TiO.sub.2 +xZrO.sub.2 .fwdarw.Ba(Ti.sub.1-x Zr.sub.x)O.sub.3 +CO.sub.2 (gas)
regions slightly richer in Zr or Ti may exist after thermal reaction reflecting the very slow diffusion of Ti and Zr atoms in the titanium dioxide sublattice of Ba(Ti, Zr)O.sub.3. These regions of slightly differing composition would still have the same crystal structure but might be characterized by slightly different lattice parameters and therefore Curie temperatures. The center of each microregion would therefore correspond to the former center of the particle which produced the enrichment of that species, as is shown in FIGS. 1a and 1b. FIG. 1a schematically illustrates a mixture of barium carbonate, zirconium dioxide, and titanium dioxide prior to thermal reaction. FIG. 1b schematically represents Ba(Ti, Zr)O.sub.3 having retained zirconium and titanium microregions after thermal reaction.
The inhomogeneous, or nonrandom, distribution of components in a particular compound such as Ba(Ti, Zr)O.sub.3 can result in electrical properties which are less than ideal. In the example cited above, the presence of regions slightly richer in Zr or Ti atoms can lead to a material having a broader and lower than desired permittivity peak since each region is characterized by a slightly different Curie temperature dependent on its average Zr concentration.
Each microregion is also characterized by a slightly different coercive electric field value and polarization value. Regions having higher Zr concentrations have lower coercive electric field values and lower polarization values, whereas the opposite is the case for microregions having higher Ti concentration may not be very great, on the order of .+-.1%, any deviation from a completely random distribution of Ti and Zr atoms in the Ti sublattice results in the macro ferroelectric ceramic part having a hysteresis loop that is less square and a polarization value lower than is optimally possible. The imposition of a distribution of polarization values and coercive voltage values in place of a distinct value, corresponding to a specific composition, degrades the performance of the ferroelectric ceramic with respect to its electrical polarization performance.
Inhomogeneities in composition which result from conventional powder processing can be eliminated by using a complete chemical processing technique which entails mixing the correct proportions of precursor ingredients in the liquid state to produce the desired composition or compound and then converting this liquid mix to a solid, usually a powder. Since mixing is performed in the liquid state, particle size effects are avoided provided the liquid can be converted into a solid without undergoing segregation. If the liquid precursors are properly chosen so as to be compatible, mixing of the components can occur at the molecular level.
The major problems inherent in all total chemical processing techniques are that they are expensive, inefficient and difficult to control. Typically only very small quantities of powder can be conveniently prepared from large quantities of liquid precursors and solvents. Since the solvents or the liquid precursors themselves are usually much more expensive than standard particulate precursors, complete chemical processing techniques are cost prohibitive on a commercial scale. More importantly, in the case of ferroelectric ceramic compounds such as Ba(Ti.sub.1-x Zr.sub.x)O.sub.3, 0&lt;x&lt;0.25, the large cation to small cation balance, or stoichiometry, is very difficult to control in the conversion of the liquid to the powder form. (Ba is considered to be a large cation and Ti and Zr are considered to be small cations.)
The accidental formation of additional crystal phases, such as TiO.sub.2 or Ba.sub.2 TiO.sub.4, due to deviations from stoichiometry incurred during the liquid to solid conversion, can severely degrade the overall properties of the final material even though good mixing of the Ti and Zr atoms in the Ti sublattice is achieved in the major phase material. The presence of excess TiO.sub.2 reduces the peak permittivity, increases the coercive voltage, and reduces the polarization of the composite ferroelectric. The presence of excess BaO or the intermediate phase, Ba.sub.2 TiO.sub.4, produces the same deterioration in properties and additionally causes the part formed from such a two-phase mixture to be moisture-sensitive.